AVS2012 Session TR+BI-TuM: Self Healing Coatings, Bio-Inspired Design, and Frictional Properties of Biological Materials

The surfaces of six types of silicone hydrogel (SH) contact lenses (PureVision®, O₂ OPTIX®, ACUVUE® Oasys ®, ACUVUE® TruEye®, Biofinity®, DAILIES TOTAL1®) and the pHEMA-based ACUVUE® 2 have been analyzed using atomic force microscopy (AFM) in aqueous environment. The elastic modulus, frictional, and adhesive properties of each lens were evaluated using calibrated instrumentations, providing a basis for comparing the distinctive surface properties of these lenses. Cantilevers modified with 5-μm (diameter) silica colloidal probes were employed throughout the experiments. Elastic modulus was measured by indenting the probe into the surface of the hydrogel in a controlled manner (i.e. approach speed and maximum applied force), such that the maximum indentation depth was restricted to sub-micron levels. A modulus value was obtained by fitting the characteristic force versus indentation behavior to a mathematical model. The frictional force was measured for the sliding contact of the probe and the surface at the length scale of 500 nm and with applied loads up 20 nN. The friction coefficient was realized by evaluating the linear dependence of friction force on applied normal load. The lenses examined exhibited an order of magnitude difference—from the softest to the stiffest sample—in modulus value, generally reflective of the distinct surface treatments they received during manufacturing. For example, the pHEMA-based ACUVUE® 2 was shown to have a modulus between 100 and 130 kPa, whereas PureVision®’s was an order of magnitude higher in value. The frictional properties of the lenses followed a similar trend in that the lenses with surface treatment, such as PureVision® and O₂OPTIX®, generally exhibited coefficients of friction five times greater than that of a non-treated lens such as ACUVUE® OASYS®. The elastic modulus and frictional properties of different lenses evaluated on a nanoscopic level by AFM depict a strong correlation between the surface treatments and the apparent mechanical behaviors of the lenses.

Cartilage is known for exceptionally low friction coefficients during sliding, but its wear resistance is arguably more remarkable. Conventional wisdom suggests that cartilage wears gradually with use and that osteoarthritis is the inevitable consequence. This notion is refuted by the scientific literature. Dissections of mature, healthy, and active joints consistently reveal smooth, glossy, damage-free articulating surfaces that can only occur if tissue recovery matches wear. Cartilage recovery is extremely slow due to a lack of vasculature and numerous lubrication mechanisms have been proposed to explain extremely low in-vivo wear rates. Osteoarthritis (OA) is characterized by progressive wear and caused by a system destabilizing input (e.g. biochemistry, acute injury, altered loading, and joint instability). In certain joint-destabilized animal models, for example, a localized defect (~50 µm wide) is visible in as little as a week, and bone-on-bone contact occurs on the order of 6 months. Recent studies suggest that interstitial lubrication, a mechanism that reduces frictional and normal stresses by nearly 100X, is the dominant protective mechanism of cartilage. Localized surface damage can disrupt the very specific structural features responsible for the unique interstitial fluid pressurization mechanism. We hypothesize that localized surface damage can initiate OA-like degradation if it is sufficiently disruptive to the interstitial lubrication mechanism. In this paper, we present friction and wear measurements designed to explore this novel mechanical hypothesis of OA initiation and progression.

There have been a significant amount of efforts to develop boundary lubrication films that have the bound and mobile natures at the same time. As an effort to develop a more efficient boundary film lubrication method, a new bound-and-mobile lubricant molecule was synthesized and its lubrication and self-healing capability was studied. Low-molecular-weight silicone molecules with cationic side groups can form bound-and-mobile boundary lubrication film on silicon oxide surface. Both nano- and macro-scale tribological tests revealed superior lubrication performance of the silicone polymer with cationic side chains (called cationic lubricant polymer, CPL) over the neutral silicone oil. The multilayer CPL films exhibited characteristic topographic features due to ionic interactions within the polymeric film. In the macro-scale, the effects of ionic content and environmental condition on self-healing will be discussed to demonstrate the wear resistance and self-healing capability. In the nanoscale, the results of disjoining pressure and viscosity measurements help understand the lateral spreading of the mobile layer and identify the mobile species. The CPL-coated surfaces are hydrophobic which prevents the detrimental effects of humidity on wear of silicon. In addition, the hygroscopic nature of CPL allows humidity to be absorbed into the film, which enhances the self-healing capabilities. By texturing the silicon surface with nanowells, self-healing can be enhanced when the nanowells are filled with CPL. The nanowells serve as CPL reservoirs that are readily available for self-healing within the wear track for faster cycle intervals. However, the nanowells deteriorate the self-healing from surrounding the contact region due to the refilling of the empty nanowells.